Three blade seal bars are commonly utilized in the thin film packaging industry for cutting a continuous ribbon of thin film packaging material (e.g., plastic or polyethylene sheeting) around a product which is conveyored to and wrapped, using known equipment, at a wrapping station. One or more seal bars are positioned at the wrapping station in a known manner, to simultaneously cut and heat seal the film, along its tracking edge and optionally along its cut length, while forming the heat seal along the leading edge of the thin film material to be wrapped around the next in line product arriving at the wrapping station.
More specifically, the three blade seal bar comprises a pair of identical blade clamping bars adapted to be clamped together with tightening screws located at spaced intervals along the length of the bars. The bars define a blade retaining channel along their length opening to the bottom edge of the seal bar. The heat seal blades with the center cutting blade therebetween project downwardly out of the channel with the center cutting blade having a blade edge projecting below the corresponding blade edges of the heat seal blades to effect cutting of the continuous packaging film. Depending upon the product being packaged, a pair of seal bars may be arranged in L-shaped configuration in plan view to simultaneously form longitudinal and transversely extending seals around the product.
It is important that the cutting and heat seal blades be properly clamped within the blade retaining channel along their entire lengths so that the respective elevational position of each blade cutting or sealing edge is constant along the entire blade length, with the cutting edge of the center blade projecting downwardly from the heat sealing blade edges. Otherwise, without the desired profiled positional and elevational relationship of the blades along their lengths, cutting and/or heat sealing of the packaging film may not properly occur, causing disruption and down time of the packaging process.
Since the blades are often slightly bowed along their length prior to insertion into the blade retaining channel formed between the clamping bars, it is of critical importance to progressively apply clamping pressure along the length of the bars while tightening the screws during clamping to ensure that the blades are properly seated within the channel in the above mentioned profiled relationship. A positioning block having an elongate, upwardly directed blade positioning slot is conventionally used to properly seat the blades within the blade retaining channel. The block is grasped by the user with one hand and the three blade edges are directed into the positioning slot beginning at one end of the blades. Manual pressure is then exerted by the user to press the blade edges firmly against the bottom of the positioning slot; this pressure is transmitted to seat the blade portions within the channel. While applying manual pressure, the user then tightens the screws which extend adjacent the positioning block. After tightening, manual pressure is then released, the block slid towards the adjacent unclamped blade portions, manual pressure reapplied and maintained while the user tightens the next set of tightening screws until the blades are clamped within the channel along their entire length.
Since manual pressure is relied upon to progressively clamp the blades within the channel along their length, any variations in manual pressure may cause the corresponding blade portions to be seated and clamped within the channel at varying elevational locations. During packaging, therefore, proper cutting and/or heat sealing of the packaging film along the blades may not properly occur.